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No Heat in 1990 GM - BDA, Heater Core, or Temp Sensor?

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    No Heat in 1990 GM - BDA, Heater Core, or Temp Sensor?

    Long-time lurker, first-time poster. I've found the guides on here helpful in the past when I needed to replace my odometer gears and rebuild my front power window motors.

    At the moment, I'm a bit puzzled about what exactly is wrong with the climate control system in my 1990 Grand Marquis. For several years now, the car has had AC which works perfectly fine, but no heat. I can move the ATC temperature selector on the dash all the way up to 85, and it stays just as cold as at 65. I used to hear a faint hiss and then a click when I moved the selector from one extreme to the other -- which I thought was the sound of the blend door actuator moving the blend door. I no longer hear that when I move the selector.

    So, I thought this was an open-and-shut case of the blend door actuator malfunctioning. Everything I read up on suggested the small gear inside the BDA unit is what almost always fails, so I bought the replacement gear, followed one of the helpful guides on here to take apart the dash for a 90-91 model, removed the BDA unit, opened it up, and... all the gears look perfect. I moved the metal gear which extends from the actuator itself with my finger to ensure that the plastic gears and the actuator arm would turn all the way from one side to the other and then back. They did.

    So I guess this means there are a few remaining possibilities:
    1. The actuator itself has somehow malfunctioned and no longer spins, in which case I suppose I should buy an all-new BDA unit.
    2. The heater core is not working properly.
    3. The in-car temperature sensor is not working properly and constantly thinks it's like 100+ degrees in the car at all times, so the heat is never activated.
    (Let me know if there's another one I'm overlooking.)

    Any thoughts on which of these is most plausible? Or any suggestions on how I should diagnose which of these is the culprit (without having to reassemble everything first)?

    Thanks for any comments!

    Edit: I should mention there was a period of a couple years when the heater "sort of" worked. It seemed like, on some occasions, when I switched the selector from 65 to 85, I could get it to switch into heat and, on other occasions, I couldn't. But once I got it to switch, it stayed on heat. This situation was also what initially caused me to believe it was something with the BDA; these symptoms seemed (to me) more consistent with a gear deteriorating than something about the temperature sensor or the heater core.
    Last edited by SomeoneWeird; 11-22-2020, 12:59 AM. Reason: added details

    #2
    Take a look under the hood. Find the 2 heater hoses and see if they are linked together and not going into the heater core itself (a.k.a heater core bypass). If the core went bad before you got it maybe someone just bypassed it since it's a PITA to replace the cores in these cars.
    These are highly engineered precision vehicles, the first step in diagnosing the problem is to strike the suspected offending part sharply and repeatedly with a blunt object, then re-test.

    Comment


      #3
      If the BDA is Good, I've heard of the blend door itself breaking. Although that may have been on later models.
      The vacuum hiss and door that you may have heard before would be mode doors or recirculation door since those are vacuum controlled while the BDA is electronic. So the absence of that hiss/click may be unrelated to the no heat issue.

      Something I've had happen is a restricted but not totally clogged heater core. The old trick of feeling if both hoses to the heater core are hot did not work in my case. They were both hot but my heat was non-existent. I found that if I ran around with the heat off for a few miles then turned the heat on, I could feel heat then it would go back to cold. The blower motor off was not removing heat from the heater core allowing it to slowly build up despite the heater core restriction. Once the blower motor was on, the heat was removed faster than the restriction allowed the core to heat up.
      Flushing the heater core would be worth a try. I have 2 old heater core hoses that I modified to connect to garden hoses. One garden hose to the water supply, the other to a large tote (sitting beside the car) to easily catch the discharge. On my '89 one flush worked for years. On my '91, it worked for maybe a month before I had to flush the core again, and then again another month later, repeating until either Winter was over or I replaced the heater core.
      Vic

      ~ 1989 MGM LS Colony Park - Large Marge
      ~ 1998 MGM LS - new DD
      ~ 1991 MGM LS "The Scab"
      ~ 1991 MGM GS "The Ice Car"

      Comment


        #4
        Move the temperature slider and be sure it’s actually moving someone under the dash. Not intimately familiar with the 90+ ATC, but I know the sliders tend to suck, and if it’s like my 88, it still moves a cable for the temperature.
        1990 Country Squire - weekend cruiser, next project
        1988 Crown Vic LTD Wagon - waiting in the wings

        GMN Box Panther History
        Box Panther Horsepower and Torque Ratings
        Box Panther Production Numbers

        Comment


          #5
          Electronic BDA for '90+ ATC equipped cars. Even the manual controls went to electronic BDA in (I think) '95, though I could be wrong on the year.
          Vic

          ~ 1989 MGM LS Colony Park - Large Marge
          ~ 1998 MGM LS - new DD
          ~ 1991 MGM LS "The Scab"
          ~ 1991 MGM GS "The Ice Car"

          Comment


            #6
            Thanks for the suggestions, everyone. Both of the hoses were indeed still connected to the core. But I (foolishly) didn't try the trick of feeling their temperature before dropping the steering column, removing the dash, etc. since I was overly-sure it was the BDA.

            Originally posted by VicCrownVic View Post
            If the BDA is Good, I've heard of the blend door itself breaking. Although that may have been on later models.
            The vacuum hiss and door that you may have heard before would be mode doors or recirculation door since those are vacuum controlled while the BDA is electronic. So the absence of that hiss/click may be unrelated to the no heat issue.

            Something I've had happen is a restricted but not totally clogged heater core. The old trick of feeling if both hoses to the heater core are hot did not work in my case. They were both hot but my heat was non-existent. I found that if I ran around with the heat off for a few miles then turned the heat on, I could feel heat then it would go back to cold. The blower motor off was not removing heat from the heater core allowing it to slowly build up despite the heater core restriction. Once the blower motor was on, the heat was removed faster than the restriction allowed the core to heat up.
            Flushing the heater core would be worth a try. I have 2 old heater core hoses that I modified to connect to garden hoses. One garden hose to the water supply, the other to a large tote (sitting beside the car) to easily catch the discharge. On my '89 one flush worked for years. On my '91, it worked for maybe a month before I had to flush the core again, and then again another month later, repeating until either Winter was over or I replaced the heater core.
            Interesting. When you were having those issues, were you having any other related issues? My (potentially incorrect) understanding had been that when heater cores start to malfunction, it typically causes other issues, too: coolant leaks, windows continuously fogging up, engine overheating issues, etc. I've had no problems like that, but if you didn't either, maybe it is my heater core after all.

            One thing I was just thinking about. You guys can tell me whether this seems reasonable or totally absurd. When I removed the BDA unit, I noticed the little metal arm guided by the BDA which moves the blend door. I grabbed it with my hand and moved it back and forth, and I could tell that it was moving the blend door since it had some weight to it and made a soft thud when I moved it all the way to one side or the other. There have been a grand total of zero days in my entire life when I've wanted to use heat and AC on the same day. I switch to heat, then six months later to AC, then six months later to heat, etc. If it's an issue with either the actuator or the sensor, couldn't I just use some duct-tape (or something with superior heat-resistance if necessary) to affix the blend door into the heat position? I noticed before removing the dash that, if I dropped the glove box, I could see the side of the BDA unit from within there -- the side that I now know makes contact with that little metal arm. So maybe I'd even be able to change its position every six months by reaching in there through the glove box and adjusting it.

            This would totally bypass both the BDA and the temp sensor, so if it's a problem with either of those, problem solved. And if the heat still doesn't work after I do this, I'll know it's the heater core for sure.

            Comment


              #7
              No other problems in my case. I got tired of flushing the heater core so decided to replace it.

              Oh, I almost forgot, make sure your engine coolant is full. Low coolant can cause no heat. (Sorry if you already mentioned doing this.)

              Fogging and coolant loss are symptoms if a leaky core. A clogged core would not have those issues.
              Vic

              ~ 1989 MGM LS Colony Park - Large Marge
              ~ 1998 MGM LS - new DD
              ~ 1991 MGM LS "The Scab"
              ~ 1991 MGM GS "The Ice Car"

              Comment


                #8
                As for the blend door, it sounds like it probably is not broken. The idea of fixing it in one position could work. Varying the temperature will not be as simple/possible but if you gain heat by doing that it could be worth the trade off until you can fix it.
                Vic

                ~ 1989 MGM LS Colony Park - Large Marge
                ~ 1998 MGM LS - new DD
                ~ 1991 MGM LS "The Scab"
                ~ 1991 MGM GS "The Ice Car"

                Comment


                  #9
                  Makes sense -- thanks again. I'll start by fixing the blend door in the heat position (hopefully I can figure out which one that is without having to try both) to see whether I get heat when I do so. If that doesn't work, I'll plan on replacing the heater core -- might as well while the dash is off.

                  Related question: Is there some trick to removing those heater core hoses? I completely unscrewed the hose clamps to the point of taking the screws fully out, but even after pulling quite hard on the hoses, they wouldn't come off. I guess I could have pulled even harder, but I was worried about damaging the hoses. I know more recent cars have a clicking mechanism for taking them off -- for the box GM, is it just a question of pulling really hard on the hoses? (I wonder if they've never been touched in 30 years and are basically just stuck at this point.) I was trying to get them off to make it easier to remove the BDA unit, but I eventually gave up and used the world's tiniest wrench in a contorted angle to do so. If I'll be swapping out the heater core, though, then I'll definitely need to remove those hoses.

                  Comment


                    #10
                    Those hoses are a pain.
                    They basically over time mold/glue/weld/bond (not sure what the correct tem is) themselves to the core inlet and outlet. The easiest way to remove them that I've found is to cut them, but if you don't have replacements on hand that might not be a reasonable suggestion. I cut starting at the end of the hose straight forward toward the front of the car. Without cutting you need to wedge something between the inlet or outlet tube and the hose. There isn't room to really do this but a right angle pick might work if a flathead screwdriver is too long. I did it this way once and it really sucked and took forever, but eventually I got the hoses to stop bonding with the tubes.

                    When putting new hoses on I put some dielectric grease on the inlet and outlet tubes. I'm not sure if that grease is ok to use with the rubber hose, but I haven't had any issues with it.
                    Vic

                    ~ 1989 MGM LS Colony Park - Large Marge
                    ~ 1998 MGM LS - new DD
                    ~ 1991 MGM LS "The Scab"
                    ~ 1991 MGM GS "The Ice Car"

                    Comment


                      #11
                      They become one over time is right. You really have to be gentle on the core as well. Its old and can become disturbed easily by wrestling with those hoses. As said above it is best to cut the hose open when removing the core. Replace with new.
                      ~David~

                      My 1987 Crown Victoria Coupe: The Brown Blob
                      My 2004 Mercedes Benz E320:The Benz

                      Originally posted by ootdega
                      My life is a long series of "nevermind" and "I guess not."

                      Originally posted by DerekTheGreat
                      But, that's just coming from me, this site's biggest pessimist. Best of luck

                      Originally posted by gadget73
                      my car starts and it has AC. Yours doesn't start and it has no AC. Seems obvious to me.




                      Comment


                        #12
                        silicone grease (dielectric grease is silicone) works fine with rubber. Thats what the stuff specified for O-ring lube actually is. It very slightly swells the rubber too, makes for a better seal, but as a bonus it makes things let go.
                        86 Lincoln Town Car (Galactica).
                        5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

                        91 Lincoln Mark VII LSC grandpa spec white and cranberry

                        1984 Lincoln Continental TurboDiesel - rolls coal

                        Originally posted by phayzer5
                        I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

                        Comment


                          #13
                          Well, unfortunately for me, it seems that I'm not getting heat even with the blend door fixed in the heat position, bypassing the BDA and the in-car temperature sensor. So it looks like it is indeed the heater core. I'm going to go ahead and buy a new one and do the replacement instead of a flush since I've taken off the whole dash already.

                          One last question before I attempt that, though. You guys mention cutting off the old heater hoses. And while I don't have new ones on hand, since I'll be buying a new heater core, I might as well buy a couple new hoses at the same time. Problem is I can't figure out which two of these hoses are the right ones: https://www.rockauto.com/en/catalog/...se+/+pipe,6892

                          Only the Motorcraft part uses the familiar inlet/outlet terminology, and they only have the Motorcraft outlet hose listed, not the inlet hose. Any idea which off-brand ones would be the right equivalents? Judging by shape/size, the Gates 18741 and Gates 18708 look like the right two, but they both say they go to "Pipe-1".

                          Comment


                            #14
                            Gates part numbers:
                            19610 goes from the heater core to the thermal blower lockout (TBL) switch.
                            18708 goes from heater core to the pipe under the passenger side of the upper intake (pipe-1, first time I've seen it called that).
                            18741 goes from pipe-1 to the water pump.
                            19683 goes from the TBL to the lower intake (front passenger side corner where the ECT sensor is.)
                            19682 can replace 19619+19683 IF you wish to eliminate the TBL.
                            The only other hose I haven't mentioned (besides upper and lower radiator hoses) is the bypass hose. Really short right angle PITA to get at hose that comes off the water pump, 20662 under cooling system.

                            Edit: also forgot there are 2 little hoses behind the upper intake for the EGR cooler. Even bigger PITA to get at with the upper intake on. Just a couple feet of plain 1/4" hose for that.
                            Last edited by VicCrownVic; 11-23-2020, 12:14 AM.
                            Vic

                            ~ 1989 MGM LS Colony Park - Large Marge
                            ~ 1998 MGM LS - new DD
                            ~ 1991 MGM LS "The Scab"
                            ~ 1991 MGM GS "The Ice Car"

                            Comment


                              #15
                              I'm not a big fan of the TBL. Usually fails and keeps the heater blower from running and it causes vacuum leaks. If you delete it, jumper the two electrical wires together and seal the vacuum connections. Ball bearings and a wee bit of silicone works nicely for this.
                              86 Lincoln Town Car (Galactica).
                              5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

                              91 Lincoln Mark VII LSC grandpa spec white and cranberry

                              1984 Lincoln Continental TurboDiesel - rolls coal

                              Originally posted by phayzer5
                              I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

                              Comment

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